For the antireflection property of surfaces, in particular of optical elements or displays, use is conventionally made of reflection-reducing interference layer systems, which contain a plurality of alternating layers made of high refractive index and low refractive index materials. Currently, use is made of MgF2 with n=1.38 as material with a particularly low refractive index in the visible spectral range. The antireflection effect of conventional dielectric layer systems could be improved if materials with a lower refractive index were available.
An alternative option for reducing the reflection of an optical element is known from the German patent document DE 10241708 B4 and U.S. counterpart publication 2005/0233083. In this method, a nanostructure, which reduces the reflection of the plastic substrate, is generated on the surface of a plastic substrate by means of a plasma etching process. The antireflection property of an optical element by generating a nanostructure on the surface thereof is advantageous in that a low reflection is obtained over a broad angle-of-incidence range.
German Patent Document DE 102008018866 A1 and U.S. counterpart publication 2011/0051246 describes a reflection-reducing interference layer system onto which an organic layer, which is provided with a nanostructure by means of a plasma etching process, is applied.
However, plasma-etched nanostructures only achieve a depth of 100 nm to 200 nm in most materials. Such a thickness suffices for planar and slightly curved surfaces for providing a substrate with such an antireflection property in the visual spectral range from 400 nm to 700 nm for angles of incidence of light between 0° (perpendicular light incidence) and 60° that the residual reflection is only approximately 1%. However, in part, there is a demand for broadband antireflection properties which are intended to work over even larger angle of incidence ranges of light.
The production of an antireflection coating on low refractive index (n<1.7), highly curved surfaces is particularly problematic. A layer deposited by a directed vacuum coating process such as sputtering or vapor deposition has a thickness at the location at which it grows which depends on the angle of the incident vapor. The layer thickness reduces with increasing angle of incidence. Therefore, the physical thickness d of all layers in an interference layer system reduces with increasing angle of incidence. However, the optical thickness n*d, where n is the refractive index, is important for the optical function. The refractive index n is different in the layer systems consisting of high refractive index and low refractive index materials such that there is an additional change in the optical function in the case of varying thickness. As a result of this problem, the residual reflection of antireflection coatings generally has undesired high values in the edge region of lenses.
An improvement could be obtained if it were possible to produce a low refractive index gradient layer with such thickness that a reduction in thickness of at least 50% is tolerated. The technical implementation on high refractive index substrates (n>1.7) is easier than on the conventional low refractive index glasses since a layer design in which the refractive index gradually reduces can already be implemented by natural materials.
There are only few technical possibilities for producing relatively thick layers with an effective refractive index <1.38. The document W. Joo, H. J. Kim and J. K. Kim, “Broadband Antireflection Coating Covering from Visible to Near Infrared Wavelengths by Using Multilayered Nanoporous Block Copolymer Films”, Langmuir 26(7), 2010, 5110-5114, describes the production of a thick gradient layer by means of sol-gel processes, wherein, however, the deposition on curved surfaces may be difficult in this case.
A vacuum-technical method for producing multilayer gradient layers is known from the document S. R. Kennedy, M. J. Brett, “Porous Broadband Antireflection Coating by Glancing Angle Deposition”, Appl Opt. 42, 4573-4579, 2003. Here, oxides or fluorides are vapor deposited onto the substrate at glancing angle. Porous layers are likewise created here as a result of shadowing effects. Thus, for this reason, the substrate needs to be positioned obliquely with respect to the angle of incidence of vapor. However, there would be additional shadowing effects as a result of the lens geometry on a strongly curved surface, and so the method cannot readily be applied to curved lenses.